Numerical investigation of the effect of multi-walled carbon nanotube additive on nozzle flow and spray behaviors of diesel fuel

Abstract

The thermal-physical properties of multi-walled carbon nanotubes (MCNTs) and MCNT blended diesel fuel were analyzed. Internal flow in injector Spray A of Engine Combustion Network and spray in a constant-volume chamber with considering the effect of the turbulence-induced breakup were modeled for pure diesel fuel and MCNT blended diesel fuel. Nozzle flow simulation showed that the addition of MCNTs into diesel fuel increased the flow turbulence at the nozzle hole outlet. Spray simulation statistically coupled with nozzle flow modeling showed that during stable spray development, there were a positive linear relationship between the liquid penetration length and the MCNT content in diesel fuel and a negative linear relationship between the Sauter mean diameter of the droplets and the MCNT content; the addition of MCNTs reduced the liquid cone angle and had an insignificant influence on the vapor penetration length.

Introduction

Recently, the use of nanoparticles in energy systems as improvers in thermal processes and catalysts for chemical reactions, especially in compression ignition engines (CIEs), has received considerable attention due to their unique thermal physical and transport properties, such as high thermal conductivity, excellent diffusion coefficient, large surface-to-volume ratio, high reactivity, and other special characteristics [1], [2], [3]. Carbon nanotubes (CNTs) are characterized by very high thermal conductivity and a very large aspect ratio compared to metallic nanoparticles and other organic nanoparticles [4]. The measured thermal conductivity coefficient of a multi-walled carbon nanotube (MCNT) in [5] is about 3000 W/m∙K at 320 K. This value of a single-walled nanotube confirmed by the experimental measurement [6] is more than 2000 W/m∙K at 300 K. Although the addition of metallic nanoparticles to traditional organic liquid fuels improves their thermal-physical properties, the by-products in chemical reactions are considered as hazards substances that release into the atmosphere and have a negative influence on the ecosystem and human health [7].

The effect of introducing CNTs to diesel fuel (DF) or biodiesel fuels on performance and emission characteristics of CIEs has been confirmed in many studies. Sadhik Basha and Anand [8] investigated a single-cylinder diesel engine running on water-emulsified DF (93 vol% DF + 5 vol% water + 2 vol% surfactants) with the addition of MCNTs (outer diameter (OD): 16 nm) in the mass fractions of 25 and 50 ppm. It was found that with increasing MCNT content, the ignition delay period decreased under all investigated engine-operating conditions. The presence of MCNTs in emulsified DF did not increase engine noise. The addition of 25 ppm MCNTs to water-emulsified DF had a negligible effect on exhaust smoke, but with increasing the MCNT mass friction to 50 ppm, exhaust smoke reduced. NO_x emissions also decreased with the presence of MCNTs. However, the effect of the amount of MCNT additive on NO_x emissions was only distinguished under high engine load conditions. This means that the temperature in the cylinder affects the efficiency of adding MCNTs. EL-Seesy et al. [9] added 50 mg/L MCNTs (OD: 10–15 nm) to DF, H20D (20% n-heptanol + 80% DF), and H40D (40% n-heptanol + 60% DF) to experimentally investigate the effect of MCNT additive on combustion process of a fast compression-expansion machine (FCEM). The addition of MCNTs into base fuels increased the maximum pressure and shortened the ignition delay period. The authors explained the reason for such effects of MCNT additive that MCNTs have a high ratio of surface area to volume and high thermal conductivity, which can improve fuel evaporation, fuel–air mixing, and combustion process. Heydari-Maleney et al. [10] dispersed 20, 60, and 100 ppm MCNTs (OD: 4–8 nm) into biodiesel blended DF - B2 (2% biodiesel from wasted vegetable oil + 98% DF) and evaluated performance characteristics of a single-cylinder natural aspirated diesel engine under full load. With the increase of MCNT content, the net power and torque monotonously decrease at engine speeds of 2300, 2900 rpm, the BTE monotonously increases and the BSFC monotonously decreases at engine speeds of 1700, 2300 rpm. MCNTs (OD: 50 nm) mixed with neem oil biodiesel in the mass fractions of 50 and 100 ppm were used to estimate the effect of MCNTs on emissions for a single-cylinder diesel engine under a constant speed of 1500 rpm [11]. The increase of MCNT content leads to a reduction of NOx, HC, and CO emissions at full engine load up to 9.2%, 7.2%, and 7.9%. A decrease in NOx and CO emissions was also noticed for a single-cylinder air-cooled diesel engine at full engine loads when MCNTs (OD 20 nm) were dispersed into B20 (20% jatropha methyl ester + 80% DF) at the dose levels of 25, 50, and 100 ppm [12]. Markov et al. [13] investigated the effect of adding different amounts of MCNTs with OD 10–15 nm (up to 500 mg/L) into pure DF on the emission characteristics of a 4-cylinder diesel-generator. The result indicated that the addition of MCNTs into DF leads to a reduction of NOx emissions by 5.3% and a sharp reduction of exhaust smoke by 57% at the maximum engine load. The emission reduction after introducing MCNTs to base fuel can be explained by the fact that the addition of MCNTs improves the quality of mixture formation and accelerates the combustion process by facilitating the decay of the fuel spray, reducing the diameter of fuel droplets, and accelerating their heating and evaporation [12], [13].

All authors of the abovementioned studies have predicted that the presence of MCNTs in liquid fuel improves the atomization and mixture formation process. However, these abovementioned studies are only qualitative analysis. The negative effect of MCNTs on the emissions of CO, HC, and NOx and exhaust smoke have also been observed in [14], [15], [16], [17], [18], [19], [20]. Therefore, it is necessary to study the processes of fuel injection, atomization, evaporation, and fuel–air mixing, which may further affect the combustion process and the performance and emission characteristics of engines [21], [22].

Some scientists and researchers have studied the evaporation of single droplets of nanofuel containing carbon nanotubes. Mei et al. [23] measured the evaporation rate of sessile droplets of nanofuels prepared by dispersing MCNTs with OD 20 nm and MCNTs with OD 50 nm, respectively, into n-tetradecane with the mass fractions of 50, 100, and 150 mg/L in base fuel. The measurement results showed that the presence of MCNTs in n-tetradecane shortened the evaporation process, and an increase in MCNT concentration or a decrease in their OD led to an increase in the average evaporation rate. The average evaporation rate of n-tetradecane blended with 150 mg/L MCNTs (OD 20 nm) was 1.84 times higher than that of pure n-tetradecane. The authors noticed that the decrease of OD and the increase of MCNT concentration make the viscosity of nanofuel be increased. The intensification of the evaporation process of neem oil biodiesel with MCNTs was also confirmed [24]. MCNTs at the concentrations of 25, 50, and 100 mg/L were added to neem oil biodiesel fuel. The increase of MCNT concentration reduced the time necessary for the complete evaporation of fuel droplets. Wang et al. [25] experimentally studied the evaporation of single droplets of biodiesel blended DF (20 wt% biodiesel + 80 wt% DF) containing MCNTs and cerium nanoparticles (CeNP) at the dose levels of 50 ppm CNTs + 50 ppm CeNP, 100 ppm CNTs + 100 ppm CeNP, 250 ppm CNTs + 250 ppm CeNP, and 500 ppm CNTs + 500 ppm. The evaporation measurement was carried out at 673 K and 873 K. It was found that there were no bubbles or micro-explosions during the lifetimes of five kinds of fuels. However, so far, the systematic study of the injection and atomization processes of MCNT blended nanofuels is extremely inadequate.

To simulate flow and thermal processes of nanofluid, researchers usually use the homogeneous single-fluid approach with assumptions of no-slip between base liquid and nanoparticles, uniform dispersion of nanoparticles, and hydrodynamic and thermal equilibrium between base liquid and nanoparticles [26]. Using this approach, Rahmaniana and Hamzavi [27] conducted 3D CFD thermal flow simulations of water-CNT nanofluid for a photovoltaic thermal system (PVT) and studied the effects of CNT concentration and nanofluid flow rate on the electrical and thermal efficiency of PVT system. A laminar 2D natural convection of water-CNT nanofluid in a shallow cavity was simulated with experimentally modified effective viscosity and thermal conductivity of the studied nanofluid [28]. The significant influence of the rheological properties of nanofluid on the flow and heat transfer was confirmed. Hayat et al. [29] numerically modeled the radiative flow of water-based nanofluid with SCNTs and MCNTs by using theoretically calculated physical and thermal properties of nanofluids. Until now, numerical studies of flow and thermal processes with the homogeneous single-fluid approach were rarely conducted for nanofuel, especially for CNTs based nanofuel.

The objective of this work is to numerically investigate the effect of MCNTs on the nozzle flow and spray behaviors of DF. Based on the result in [13], there is a potential to continue increasing the MCNT content in DF from the point of view of reducing exhaust smoke. So in this study, the MCNT content in DF was raised to 1000 mg/L. The homogeneous single-fluid approach was introduced to model the injection and spray of MCNT blended DF. Using theoretical models, we calculated the thermophysical properties of MCNT blended DF. The transient nozzle flow of MCNT blended DF during injection progress was simulated for a single-hole injector Spray A of Engine Combustion Network (ECN). The spray simulations of MCNT blended DF coupled with injection flow modeling were conducted in a cylindrical constant-volume chamber.